Toner image fixing of some receiving sheets is plagued with the problem of blistering. In most environments, paper based receiving sheets carrying toner images have a certain amount of moisture. When that moisture is heated to 100.degree. C. or greater, it has a tendency to turn into steam, greatly expanding and escaping the paper. Ordinary bond paper is sufficiently porous to allow the escape of the moisture without damage to the paper in ordinary toner fusing. However, paper that is more highly finished often has less porous outside layers as a result of that finishing which block the escape of steam. The steam pushes the outside layer away from the substrate of the receiving sheet creating a blister.
U.S. Pat. No. 5,089,363, issued Feb. 18, 1992 in the name of Rimai et al and U.S. Pat. No. 5,023,038, issued Jun. 11, 1991 in the name of Aslam et al, disclose a method of fixing a multicolor toner image carried on a heat softenable, for example, thermoplastic, outside layer of a receiving sheet. The receiving sheet is passed across a preheating plate to raise the temperature of the heat softenable layer to or above its softening point. It is fed into a pressure nip created by a pressure roller and a belt or web backed by a heated roller. The belt or web is of a hard ferrotyping material such as stainless steel, nickel, or the like. Relatively high pressure is applied between the belt and pressure roller to embed much of the toner image in the heat softenable layer fixing the image. Some of the toner may not be entirely embedded but may be fused on the top layer, but with much of it embedded, the hard ferrotyping belt provides photographic quality with an absence of relief and a high gloss. The toner image and heat softenable layer remain in contact with the belt as it moves away from the pressure nip. The belt and sheet are allowed to cool until the heat softenable layer and toner image are below their glass transition temperatures. At this point the sheet can be separated without offset. All this is accomplished without the use of offset preventing liquids which would reduce the quality of the image. This method is especially useful for very small particle color toner images.
The preheating step and the above process has a tendency to drive some of the moisture out of the paper before the receiving sheet enters the nip. This process can be gentle enough to not cause blistering. However, if moisture remains in the paper in the nip, it is constrained by the nip from escaping and then may blister the sheet as the sheet leaves the nip.
Conventional fixing without the benefit of the heat softenable layer but still using a finished receiving sheet that has a tendency to blister may actually present more problems than the materials in the above prior applications. Without the heat softenable layer it may be necessary to raise a high quality toner to a temperature of between 120.degree. and 150.degree. C. to accomplish fixing. This is done over a substantially wider pressure nip which constrains the receiving sheet over a longer period of time without allowing the escape of steam. Again, when the sheet leaves the nip the constrained moisture which is now turned to steam escapes more rapidly than the porosity of the receiving sheet layers can handle and the sheet blisters.
U.S. Pat. No. 4,963,943 to Tamary issued Oct. 16, 1990 suggests cooling the pressure roller (contacting the back of the sheet) in response to a fixing apparatus entering a "standby" mode which cooling step is deactivated when the apparatus goes back into a "run" mode. This approach prevents overheating of the fusing roller (contacting the image) which among other benefits reduces the tendency of the receiving sheets to blister. See also U.S. Pat. Nos. 3,993,124; 4,082,137; 4,085,794 and 4,092,099.